Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A surgeon introduces a catheter assembly having a balloon portion percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The surgeon advances the catheter assembly through the coronary vasculature until the balloon portion crosses the occlusive lesion. Once in position, the surgeon inflates the balloon to radially compress the atherosclerotic plaque of the lesion and remodel the vessel wall. The surgeon then deflates the balloon to remove the catheter.
An advance on PTCA involved using an intravascular stent. Mechanically, stents act as scaffoldings, physically holding open and, if desired, expanding the vessel wall. Typically, stents compress for insertion through small vessels and then expand to a larger diameter once in position. U.S. Pat. No. 4,733,665, issued to Palmaz; U.S. Pat. No. 4,800,882, issued to Gianturco; and U.S. Pat. No. 4,886,062, issued to Wiktor disclose examples of PTCA stents.
Before this procedure can occur, equipment for the procedure must be manufactured. Stent crimping is a critical step in manufacturing this equipment in that stent retention depends on it. Generally, stent crimping is the act of affixing the stent to the delivery catheter or delivery balloon so that it remains affixed to the catheter or balloon until the physician desires to deliver the stent at the treatment site. Current stent crimping technology is sophisticated. A short time ago, one process used a roll crimper. This damaged many polymer coatings due to its inherent shearing action. Next came the collet crimper; in it, metal jaws are mounted into what is essentially a drill chuck. The jaws move in a purely radial direction. This movement was not expected to shear the coating, because it applied forces only normal to the stent surface. But some stent geometries require that stent struts scissor together during crimping. In those geometries, even if the crimper imposes only normal forces, the scissor action of the stent struts imparts shear. Finally, the iris or sliding-wedge crimper imparts mostly normal forces with some amount of tangential shear.
To use a roll crimper, first the stent is slid loosely onto the balloon portion of the catheter. This assembly is placed between the plates of the roll crimper. With an automated roll crimper, the plates come together and apply a specified amount of force. They then move back and forth a set distance in a direction that is perpendicular to the catheter. The catheter rolls back and forth under this motion, and the diameter of the stent is reduced. The process can be broken down into more than one step, each with its own level of force, translational distance, and number of cycles. With regard to a stent with a drug eluting coating, this process imparts a great deal of shear to the stent in a direction perpendicular to the catheter or catheter wall. Furthermore, as the stent is crimped, there is additional relative motion between the stent surface and the crimping plates. As a result, this crimping process tends to damage the drug eluting stent coating.
The collet crimper is equally conceptually simple. A standard drill-chuck collet is equipped with several pie-piece-shaped jaws. These jaws move in a radial direction as an outer ring is turned. To use this crimper, a stent is loosely placed onto the balloon portion of a catheter and inserted in the center space between the jaws. Turning the outer ring causes the jaws to move inward. An issue with this device is determining or designing the crimping endpoint. One scheme is to engineer the jaws so that when they completely close, they touch and a center hole of a known diameter remains. Using this approach, turning the collet onto the collet stops crimps the stent to the known outer diameter. While this seems ideal, it can lead to problems. Stent struts have a tolerance on their thickness. Additionally, the process of folding noncompliant balloons is not exactly reproducible. Consequently, the collet crimper exerts a different amount of force on each stent in order to achieve the same final dimension. Unless this force, and the final crimped diameter, is carefully chosen, the variability of the stent and balloon dimensions can yield stent coating or balloon damage.
Furthermore, although the collet jaws move in a radial direction, they move closer together as they crimp. This action, combined with the scissoring motion of the struts, imparts tangential shear on the coatings that can also lead to damage. Lastly, the actual contact surfaces of the collet crimper are the jaw tips. These surfaces are quite small, and only form a cylindrical surface at the final point of crimping. Before that point, the load being applied to the stent surface is discontinuous.
In the sliding wedge or iris crimper, adjacent pie-piece-shaped sections move inward and twist, much like the leaves in a camera aperture. This crimper can be engineered to have two different types of endpoints. It can stop at a final diameter, or it can apply a fixed force and allow the final diameter to float. From the discussion on the collet crimper, there are advantages in applying a fixed level of force as variabilities in strut and balloon dimension will not change the crimping force. The sliding wedges impart primarily normal forces, which are the least damaging to stent coatings. As the wedges slide over each other, they impart some tangential force. But the shear damage is frequently equal to or less than that of the collet crimper. Lastly, the sliding wedge crimper presents a nearly cylindrical inner surface to the stent, even as it crimps. This means the crimping loads are distributed over the entire outer surface of the stent.
All current stent crimping methods were developed for all-metal stents. Stent metals, such as stainless steel, are durable and can take abuse. When crimping was too severe, it usually damaged the underlying balloon, not the stent. But polymeric coatings present different challenges.
Moreover, as part of polymeric stent manufacture, brittle polymeric material is laser cut. The polymer's brittle nature and the stress induced by laser cutting often causes stress cracking in the polymeric stent.
In the drug eluting stent arena, drugs are commonly placed on the stent in combination with a polymer or mixed into the polymer for polymeric stents. This placement typically coats all stent surfaces or causes the drug to be distributed throughout the polymeric stent. Then the stent is crimped onto the catheter. In general, polymer coatings are softer, weaker, and less durable than the underlying stent material. Upon crimping with a sliding wedge crimper, and following crimp protocols for the particular stent, coating damage is frequently seen. FIGS. 1 and 2 show an Elasteon 80A (a polyurethane) coating on poly(ethylene-co-vinyl alcohol) (EVAL) after crimp, grip, and the wet expansion test.
Grip is a process conducted after crimping to further increase stent retention. An outer sleeve restrains the crimped stent. Simultaneously, pressure and heat are applied to the stent-balloon section. Under this action, the balloon material deforms slightly, moving in between the struts. In a wet expansion test, the final stent-on-catheter assembly is immersed in deionized water at 37° C. for 30 seconds. Then the balloon is inflated according to the device instructions to at least a nominal pressure (8 atmospheres). After holding this pressure for 30 seconds, the balloon is deflated, and the stent slides off. After drying, the stent can be examined by optical microscopy or scanning electron microscopy for coating damage.
The primary purpose of the polymer in the stent coating is to contain the drug and control its release at a desired rate. Other obvious specifications for the polymer are a high level of vascular biocompatibility and the ability to flex and elongate to accommodate stent expansion without cracking or peeling. Meeting all of these objectives, while also possessing a high level of toughness and strength to withstand conventional crimping process, can be challenging.
A crimping process that minimizes damage to the polymer coatings of stents is needed. Moreover, a crimping process that minimizes internal stress or strain in the polymeric substrate of a polymeric stent is also needed.